Synthetic quartz glass preform

ABSTRACT

The invention relates to a synthetic quartz glass preform which is produced according to the flame hydrolysis technique with subsequent cooling and is suitable for the application of high-energy DUV radiation in the wave length range under 250 nm. Said preform has a core area which contains ≧1150 ppm OH, a strain double refraction of ≦5 nm/cm and a resistance to high-energy DUV radiation as a result of a transmission reduction of Δ T ≦0.1 %/cm thickness. The quartz glass has been exposed to the following radiation: wavelength λ 1 =248 nm, laser shot frequency ≧300 Hz, laser shot value ≧10 9  and rumination ≦10 mJ/cm 2 , and wavelength λ 2 =193 nm, laser shot frequency ≧300 Hz, laser shot value ≧10 9  and rumination ≦5 mJ/cm 2 . A device for producing said preform comprises a horizontally positioned muffle with two different-sized openings facing each other. The larger of said openings is for removing the preform, the smaller opening being for introducing a burner. The internal chamber of the muffle narrows from the larger opening to the smaller opening.

BACKGROUND OF THE INVENTION

The invention relates to a synthetic quartz glass preform and a devicefor producing the same.

In accordance with developments in the semiconductor industry, andutilization of the products from the semiconductor industry in variousfields of application, as well as due to independent developments, inparticular in the special fields of materials management and medicine,light sources with very high energy densities find application.Particularly, these are excimer laser with operation wavelengths of 248nm and 193 nm. The optical components used thereby for imaging anddirecting the radiation, as well as the photomasks which exclusivelyconsist of synthetic quartz glass or calcium fluoride have to satisfythe required optical quality and must not lose the same in continuousoperation. The most important high-quality features of the opticalcomponents arid the most difficult ones to be set, are opticalhomogeneity and stability with respect to excimer laser irradiation inthe deep ultra-violet light (DUV). Therefore, there was no lack oftrials in the past to obtain such high-quality features permanently andreproducibly.

Hence, there is known a method for producing a homogeneous, striae freebody of quartz glass from DE 42 04 406 A1, in which a rod-shaped initialbody is twisted, multiply thermally remodeled in a mould of suitableforeign material and twisted again. In the EP 0 673 888 A1 this methodis modified under avoidance of any contact with a foreign material insuch a manner that a quartz glass body subsequently produced accordingto the method is optically homogeneous in three directions andadditionally is stable with respect to excimer laser radiation. However,EP 0 673 888 A1 does not teach to which degree this stability isachieved. Additionally, the method is considerably time and costconsuming.

Synthetic quartz glass is characterized by having very good transmissionin the deep range of ultra-violet light (DUV). When it is exposed tohigh energy short-wave radiation as, for example, provided by excimerlasers at 248 nm and 193 nm, photochemical reactions will result, whichwill lead to the formation of paramagnetic defects, the latter beingresponsible for the formation of absorption bands and the development ofluminescence. The power of these photochemical reactions depends onintrinsic defects in the form of binding anomalies. The photochemicalreactions are also intensified by contaminants in a network as given,for example, by atoms of transition metals and chlorine. Parallel tothese photochemical reactions which impair the optical properties of thequartz glass, annealing processes take place for which an OH content anda content of free hydrogen in the quartz glass is of importance.

From the subsequently discussed prior art it is known to desensitizesynthetic quartz glass to high-energy radiation in the DUV by thefollowing measures, carried out individually or in combination:introducing molecular hydrogen into the quartz glass bulk, usingparticularly pure starting raw material, using chlorine free startingraw material, and doping the quartz glass with fluorine and others. TheEP 0 483 752 A1 ( U.S. Pat. No. 5,410,428) reference relates to asynthetic silica glass with a content of molecular hydrogen of at least5·10¹⁶ molecules/cm³ which is manufactured by a process wherein a quartzglass body is exposed to a hydrogen atmosphere in a furnace at a hightemperature and a high pressure for a defined time, until a desiredhydrogen concentration has been established in its interior;subsequently the silica glass body is definedly cooled down to ambienttemperature. This silica glass is known as being very stable againsthigh-energy radiation in the DUV, although it has only been exposed to2·10⁶ laser shots. It is disadvantageous that an after treatment of thesilica glass is necessary including extensive safety measures requiredthereto. Furthermore, the produced silica glass bodies exhibiting thedesired properties may not be of very large volume.

The EP 0 525 984 A1 reference describes a method for producing quartzglass which is adapted to be exposed to an excimer laser irradiation.However, the resistance property of the same is only disclosed up to alaser shot rate of about 10⁶ at an energy density of 200 mJ/cm², a shotfrequency of 100 Hz and a wavelength of λ=193 nm. The method does notfunction without a specific homogenizing step which renders itexpensive.

The patent specification EP 0 737 654 A1 relates to a synthetic quartzglass with a content of molecular hydrogen of at least 10¹⁸molecules/cm³ and a low OH content of a maximum of 50 ppm, which at atemperature of maximally 500° C. and under a high pressure is enrichedwith H₂. The stability is specified with 1.3·10⁷ laser shots at anenergy density of 350 mJ/cm², a shot frequency of 400 Hz and awavelength of 248 nm, Also in this case, a subsequent treatment of thequartz glass is required, to which end a chlorine free raw material canbe used.

In U.S. Pat. No. 5,364,433 a synthetic quartz glass suited forproduction of DUV-stepper lenses and a method for producing the same isdisclosed. The quartz glass exhibits an OH content of 10-100 ppm, achlorine content of maximally 200 ppm, a molecular hydrogen content of<10¹⁶ molecules/cm³, a refractive index homogeneity of >5·10⁻⁶ and astrain of >5 nm/cm. The stability of this quartz glass against excimerlaser irradiation at a low absorption is only disclosed up to a low 10shot rate of 0.8·10⁶ (energy density 200 mJ/cm², shot frequency 100 Hz,λ=193 nm). The comparatively low stability is explained in that adehydration step provided for in the manufacturing process leads to anincrease of the Cl content which, in turn, reduces the DUV stability. Anadditionally provided homogenizing step renders the method moreexpensive.

A substrate plate for photomasks which shows a H₂ content between 10¹⁷and 10¹⁹ molecules/cm³ is disclosed in EP 0 636 586 A1. This solution islittle or not at all suited for the production of imaging opticalmembers in the DUV range, which are subject to considerably higherrequirements with regard to the transmission and the optical homogeneitythan photomasks.

U.S. Pat. No. 5,086,352 discloses optical components made of syntheticquartz glass which can be employed in DUV excimer laser irradiation anda method for producing the same. The optical components exhibit an OHconcentration of at least 100 ppm and a doped hydrogen concentration ofat least 5·10¹⁶ molecules/cm³ (and an amount of 1·10²⁰ molecules/cm³released at degassing, respectively,) and are free from stratificationin at least one direction. 50 ppb are given for the chemical purity ofthe component in the most pretentious case for Na, K and LI, and 10 ppbfor Mg, Ca, Ti, Cr, Fe, Ni, and Cu. The preforms of the opticalcomponents are characterized in that there is no stratification parallelto the incident light, that the OH concentration rises from a centralminimum to a maximum without a point of inflection, that in the rangebetween minimum and maximum the refractive index inhomogeneity is 2·10⁻⁶or lower, and in that there exists a hydrogen doping. Such a preformshows profiles of the OH concentration, of the Cl concentration and of afictitious temperature which are to be adjusted for obtaining a highrefractive index of homogeneity. The method for producing the opticalcomponents comprises, in each case, steps for removing stratificationsand for doping with hydrogen which renders the entire production processcomplicated and expensive. Moreover, the stability is only given up to acomparatively low laser shot rate of 10⁷ (energy densities: 400 and 100mJ/cm², respectively, shot frequencies: 100 Hz, λ: 248 and 193 nm,respectively,).

U.S. Pat. No. 5,325,230 is based on U.S. Pat. No. 5,086,352 andadditionally requires for the optical component of synthetic quartzglass an absence of oxygen defects and a strain birefringence, which hasto be <5 nm/cm. The OH concentration distribution is axiallysymmetrical. Also in this case, stratifications have to be removed anddoping with hydrogen has to be carried out in the course ofmanufacturing the optical components. Considerable and expensive effortshave to be made to obtain a high purity of the quartz glass, which alsofinds expression in that special measures have to be taken for storingthe basic materials.

In the EP 0 747 327 A1 reference, a method for the heat treatment andconsolidation of a quartz glass preform is described whereby a reductionof the laser induced defects in the quartz glass is asserted to beobtained. There is nothing reported of the refractive index homogeneity,of the form and mass of the bodies to be produced, of a feasibleapplication of the produced quartz glass under extreme conditions. Therepresented increases in absorption at 248 nm and 193 nm, respectively,are only acceptable up to few million shots.

EP 0 622 340 A1 discloses an improved method for producing a body ofsynthetic silica glass. A burner comprising at least five nozzles issupplied with fuel gas in such a manner that the produced syntheticsilica glass shows an OH content optimized compared to the H₂ content.There is nothing reported with respect to the DUV stability and therefractive index homogeneity. For obtaining OH contents above 1150 ppm,this procedure is unstable with respect to the attainable growthbehavior.

In EP 0720 969 A1, a quartz glass, an optical component containing thisquartz glass and a manufacturing process for the quartz glass aredescribed. For the production of the preforms a downward directedburner. is employed. The stability of the quartz glass with respect tothe excimer laser irradiation lies at a comparatively low shot rate ofabout 10⁶. A Cl content of 10 ppm is achieved by an extremely lowuneconomical raw material feed of 70 g/min·cm² via the central nozzle ofthe burner. The OH concentration of the quartz glass substantially liesat only 900 ppm.

In EP 0 720 970 A1 there is described a quartz glass forphotolithographic applications, an optical component containing saidquartz glass, a photolithographic device containing said component, anda method for producing the quartz glass. There are conditions disclosedfor producing a preform which can also be utilized in the DUV. However,the stability of the quartz glass against excimer laser irradiation isonly represented up to 10⁶ shots. The quartz glass is subjected to anF-doping which ensures, as known, low dispersion losses and has afavorable effect on the DUV stability. However, a high opticalhomogeneity of the melted quartz glass will not. be attainable owing tothe F-doping. In the course of SiO₂ deposit on places with the highesttemperature, there also develop the highest OH concentration and thehighest F concentration. Hence, an error is introduced which increasesthe gradient of the refractive index curve.

Finally, EP 0 735 006 describes a method for producing quartz glass inwhich the growth process of the quartz glass produced syntheticallytakes place in an upright direction. The process is controlled in such amanner that the stratification is adapted to take place only verticallyto the growth direction of the preform.

SUMMARY OF THE INVENTION

By virtue of the present invention the disadvantages of the priorsynthetic quartz glasses are obviated which, up to now, did not permitthe utilization of the same in extreme applications in the DUV.Therefore, it is an object of the present invention to produce, underuse of a flame hydrolysis technique, a synthetic quartz glass whichmeets highest requirements concerning stability with respect to excimerlaser irradiation in the DUV at a high energy density and concerningoptical homogeneity. It is a further object of present invention toprovide a device which is particularly suited in the manufacture of thequartz glass and which renders the output of the manufacturing processmaximal.

According to the present invention, the objects are realized by thecharacteristic features described herein. It is also feasible to employradiation of other wavelengths, provided that the same lies under 250nm. The excitation conditions can be varied; for example, a transmissionreduction of ΔT<0.05%, is obtained at a laser shot frequency (frequency)of >400 Hz, a laser shot rate (shot number) of >10⁸, and an energydensity <25 mJ/cm² with respect to the wavelength λ₁=248 nm. Thetransmission reduction corresponds to damage behavior for values statedherein. Hence, it lies within the scope of the invention. It can begenerally stated that a varied reduction of the internal transmissiontakes place at a radiation variation, but an unchanged damage behavior.Damage behavior is to be understood by someone skilled in the art as along-term damage, for example, a transmission variation of syntheticquartz glass under the effect of an excimer laser irradiation.

A core area of the preform extends over at least 50 to 90% of thepreform diameter, which can amount to up to 18 cm and more. It showsneither an axial stratification nor a stratification at right angles toits direction of growth; its entire volume is free from stratifications.The growth range of the drum-shaped preform has an at least almost flatpart close to the center, which substantially conforms to the core, anda peripheral part with a parabolic face which passes over into acylindrical surface of the drum-shaped preform. The cross-sectional areaof the preform which can be utilized for different purposes and in whichthe quality of the synthetic quartz meets the respective requirements isdifferent. Thus, for example, it is sufficient when used in illuminationsystems for excimer laser, that the synthetic quartz glass has highstability and transmission at an adequate homogeneity. In this case, 70%to 90% of the inner cross-section of the preform can be utilized. Whenprojection elements for directing high-energy laser irradiation are madefrom the preform, then under the same conditions a limitation to theinner 50 to 70% of the cross-section of the preform is necessary.Thereby, it is essential that across the inner cross-section of thepreform not only high stability and transmission exist, but also highhomogeneity; this means, however, that the OH content of the preform isconstant to ±10 ppm over this inner cross-section. Advantageously, theOH content of the core area of the preform amounts to at least 1250 ppmat a tolerance of ±10 ppm. The Cl content of the same does not exceed 20ppm and preferably is 5 to 15 ppm. The H₂ content of the core area ofthe preform advantageously amount to >1·10¹⁸ or molecules/cm³. A preformhaving the abovementioned parameters is, to a high degree, stableagainst high energy DUV irradiation, shows a high refractive indexstability and is excellent for the production of optical members such asDUV stepper-lenses, directing members for laser beams, photomasks etc.At least over a part of the core area, the preform advantageouslyexhibits a refractive index homogeneity of <0.5·10⁻⁶. Thereby, traces ofcontaminating elements (e.g. Cr, Co, Fe, Ni, Cu, V, Zn, Al, Li, K, Na)can be contained in the preform up to 500 ppb. The preform does notrequire any additional doping with H₂, F and others, to render itserviceable for tasks in DUV excimer laser irradiation. Also asubsequent treatment of the synthetic quartz glass in a reducingatmosphere is not required. If necessary, it is advantageous to cutoptical members out of the material of the core area.

A device for producing the preform comprises a substantially horizontalmuffle with two differently sized arranged openings opposing each other,the larger of which is adapted for inserting the preform and the smallerone for inserting a burner, and an internal chamber which narrows fromthe larger opening to the smaller opening. The burner is provided withnozzles which are coaxially arranged to each other and to the burneraxis, the centrally arranged nozzle discharges the basic material, forexample, SiCl₄ and O₂ and the external nozzles the fuel gas, forexample, H₂ and O₂, parallel to one another and to the burner axis. Thenarrowing substantially is a gradual one. Unlike similar prior devices,the muffle has neither an opening nor a bulge on its top-side. Theoverall length of the muffle is at least twice the size of the diameterof the vitreous preform. The almost planar leading face of the latter ispreferably arranged in the center of the internal chamber: of themuffle. The muffle is preferably embodied in three layers in order toensure a sufficient and constant internal temperature as well as a lowheat emission. It is advantageous, when the distance of thesubstantially rotation-symmetrical preform surface relative to theinternal limiting face is 5 to 100 mm depending on the flow conditionsfor the waste gas. Furthermore, it is advantageous, when the distance ofthe burner to the preform is 135 to 350 mm in dependence on the geometryof the burner nozzles and the flow of the fuel gases volume. As to thesmaller opening, in which the burner is arranged for free movement, adiameter of 50 to 100 mm is to be recommended.

Due to the internal geometry of the muffle and the operation of theburner, the device of the present invention ensures that the preform isdefinedly distributed by the fuel gas as well as that preforms ofprincipally optional lengths can be the melted on. No subsequenttreatment (twisting, doping) is required for the preform. A modificationof geometry in order to adapt the preform to the intended applicationcan be combined with a heating of the preform. In spite of extremeprocess control the device permits melting preform masses of 50 kg andmore, in a normal melting process, which are optically homogeneous andstable in the DUV against high-energy laser beams.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained hereinafter in more detail by virtue ofthe schematic drawings. There is shown in:

FIG. 1 a device for melting on a preform,

FIG. 2 a diagram in which internal transmission of a quartz glass block,cut out of a preform, is plotted as a function of laser pulse rate at awavelength of λ₁=248 mn,

FIG. 3 a diagram corresponding to that of FIG. 2 for λ₂=193 nm,

FIG. 4 a top view of a preform, and

FIGS 5 a-5 d four graphs obtained from a laser induced fluorescence(LIF) at a transmission with laser light of the wavelengths λ₁=248 nmand λ₂=193 nm, from the OH content and the strain-induced birefringence.

DETAILED DESCRIPTION

In FIG. 1, a horizontally arranged muffle 10, which effects low heatlosses caused by radiation, is represented by three shells 11, 12, 13,wherein the shell 11 is a high porosity insulating material, forexample, a ceramic fiber material, shell 12 is a fire-resisting concreteor chamotte, and shell 13 is a high temperature resistant material, forexample, Al₂O₃ or SiC. The muffle 10 has an internal chamber 14 with asmaller opening 15 for inserting a burner 16 and a larger opening 17 viawhich a preform 18 to be melted on protrudes into the muffle 10, thegeometrical axis of said preform coincides with an axis of rotation X—X.At least a portion of the muffle 10 which envelopes the preform 18 is atleast approximately symmetrical about the axis X—X, too. There is aspace a between a parabolic face 19 of the preform 18 and a limitingsurface 20 of the internal chamber 14. The space a advantageously is notlarger than 50 mm and not smaller than 15 mm in order to eliminatedeposits on the limiting surface 20 by the material to be melted. In theinternal chamber 14, the preform 18 is provided with a cap 21 having asubstantially planar leading face of which a plateau 22 lies in thecenter of the muffle 10 and is at right angles to the axis X—X. Theparabolic lateral face 19 of the preform 18 is a side of the cap 21. Viathe opening 15, which in the present embodiment has a diameter of 60 mm,the burner 16 is inserted into that portion of the muffle 10 whichdeviates from the axial symmetry, in such a manner that its axis Y—Y isslightly inclined relative to the axis of rotation X—X and intersectsthe plateau 22 below the intersection point of the axis of rotation X—Xand the plateau 22. The burner 16 is provided with a plurality ofnozzles, not shown in detail, which are in parallel to each other. Acentrally arranged nozzle discharges 410 g/min·cm² SiCl₄ and nozzlesarranged peripherally to the former, discharge 14.5 m³/h O₂ as well as 7m³/h O₂ so that a growth rate of 8 mm/h results. The burner 16 isadjustable within the opening 15. The torch 23 of the burner 16 isdirected towards the plateau 22.

In a method for producing the preform 18 from synthetic quartz glass,which principally is taught, for example, in DE 42 03 287 C2, SiO₂particles are formed from SiCl₄ by means of an H₂/O₂ flame andimmediately vitreously melted on at temperatures of over 2000° C. toyield the drum-shaped, glassy preform 18. The preform 18 has an axiallysymmetrical refractive index profile. The preform 18 is taken from thearrangement after completion of the melting process and is subjected toa conventional cooling process in order to reduce internal strains to <5nm/cm strain birefringence. The preform 18 does not show anystratifications. By virtue of the arrangement described hereinbefore,the drum-shaped preform 18 is produced, the synthetic quartz glass ofwhich exhibits the abovementioned parameters concerning the OH contentand the Cl content, as well as the internal transmission and theoutstanding low decrease of transmission under the specified radiationconditions as well as a high optical homogeneity. They are specified inthe following figures.

In the Cartesian coordinate system of FIG. 2 laser pulse numbers 100 upto 1100, multiplied by 10⁶ are plotted along the x-coordinate and theinternal transmission in % is plotted along the y-coordinate at athickness of the glass layer of 10 mm. A curve 1 represents the internaltransmission T_(i) for laser light of the wavelength 248 nm for thequartz glass; which is very high at 99.84% and is constant up to 700·10⁶laser pulses. Only then it slightly slopes, namely by 0.02%, up to1100·10⁶ laser pulses. Accordingly, the decrease in transmission ΔT liesat 900·10⁶ laser pulses far below the value of 0.1% mentioned above. Thefurther conditions are: laser frequency=300 Hz, energy density=10mJ/cm².

As to the Cartesian coordinate system of FIG. 3 the same measures arevalid as in FIG. 2. The quartz glass of 10 mm thickness is exposed to alaser irradiation of the following conditions: λ₂=193 nm, laserfrequency=300 Hz, energy density=1.5 mJ/cm². Curve 2 represents theinternal transmission of the quartz glass, which is constant up to300·10⁶ laser pulses, and which decreases by 0.05% between 300·10⁶ and500·10⁶ laser pulses, and between 500·10⁶ laser pulses and 700·10⁶ laserpulses by 0.04%, and then remains constant up to 1100·10⁶ laser pulses.Also in this case, the condition for the transmission decrease ΔT<0.1%/cm is maintained.

In FIG. 4 a section of a plan view of a preform 18 is represented with aradius r=6 cm. The radius vector r is the x-coordinate in the followingFIG. 5. Thereby, the excitation conditions for the laser irradiation asmentioned in FIG. 2 and 3 are also valid.

In FIG. 5a the LIF values for the wavelength λ₁, are plotted in a curve3 as a function of the radius by centimeters. In order to determine theLIF values, results of research for determining the parameters effectingthe long-term stability of optical components under the effect of laserradiation, as published in W. Triebel et al. in the Journal TechnischesMessen, vol. 63 (1996), number 7/8, pp. 291-295 have been utilized. Thestate of an unchanged luminescence was obtained after 2000 laser shots,in measuring an unchanged luminescence at a wavelength of 650 nm. TheLIF values of 0.7 to 2.5 determined via the radius lie at about{fraction (1/10)} of the LIF values of the relevant products of theprior art. The same is valid for the LIF values of the wavelength λ₂,which in FIG. 5b are plotted in a curve 6, centimeter by centimeter ofthe radius and, from the center up to the edge of the preform 18, rangefrom 0.55 to 1.8.

In FIG. 5c the OH content detected via the radius is plotted centimeterby centimeter in a curve 4. Thereby it becomes obvious that the OHcontent in a core area of 4 cm exceeds by far a minimum value of 1150ppm and that the minimum value, even at the periphery of the preform 18,is set at 1180 ppm.

In FIG. 5d the values of the strain-induced birefringence (SDB) measuredas a function of the radius are plotted in a curve 5. There it becomesobvious that the values, at least in the core area up to 4 cm, fall farbelow the limiting value of 5 nm/cm layer thickness for thestrain-induced birefringence and that in the edge portion (r=5 to 6 cm)of the preform 18 this limiting value is at least almost maintained.

All features disclosed in the specification and in the drawings aresubstantial for the invention both, individually and in any combinationwith one another.

What is claimed is:
 1. A synthetic quartz glass preform for applicationof high-energy DUV radiation having a wavelength of less than 250 nm,comprising: a core area having an OH content of ≧1150 ppm, a strainbirefringence of ≦5 nm/cm, an H₂ content of ≧1·10¹⁸ molecules/cm³, a Clcontent of ≦20 ppm, and a total concentration of elements Cr, Cb, Fe,Ni, Cu, V, Zn, Al, Li, K, Na of 500 ppb or less; said core area beingsubstantially free of stratifications; said core area having a stabilitywith respect to high-energy DUV radiation that is defined by atransmission reduction of ΔT≦0.1%/cm thickness, when the syntheticquartz glass preform is subjected to radiation at a wavelength λ₁=248nm, a laser shot frequency ≧300 Hz, a laser shot number ≧10⁹, and anenergy density ≦10 mJ/cm², and when the synthetic quartz glass preformis subjected to radiation at a wavelength λ₁=193 nm, a laser shotfrequency ≧300 Hz, a laser shot number ≧2 10⁹, and an energy density ≦5mJ/cm²; and said preform being produced by a method comprising the stepsof: applying a flame combusting a fuel comprising H and O to a Sicompound to form the preform without doping with F, and cooling thepreform.
 2. The preform as claimed in claim 1, wherein at a radiationwavelength variation applied to the preform a varied transmissionreduction through the preform takes place and said preform also hasunchanged luminescence.
 3. The preform as claimed in claim 2, whereinsaid preform has a preform diameter and the core area has a core areadiameter, said core area diameter being at least 50% of the preformdiameter.
 4. The preform as claimed in claim 3, wherein the OH contentof the core area is ≧1250 ppm.
 5. The preform as claimed in claim 1 or4, wherein the OH content of the core area is uniform at a tolerance of±10 ppm.
 6. The preform as claimed in any of claims 1, 2, 3 or whereinat least a portion of the core area exhibits a refractive indexhomogeneity of >0.5·10⁻⁶.